DCL says,
The following age and height requirements apply to pools on Disney ships:
Quiet Cove Pool: Reserved for Guests 18 and older
Mickey's Pool: Designed for children ages 3 years of age and older (swim diapers are prohibited) Donald's Pool and Goofy's Pool: Designed for younger Guests and families (swim diapers are prohibited).
Whirlpool Spa tubs located by Donald's Pool and Goofy's Pool: Adult supervision required for guests under the age of 16.
Mickey's Slide: Children must be between 4 and 14 years old and no greater than 64 inches in height.
AquaDuck on the Disney Dream and the Disney Fantasy: Designed for adults and children who are at least 42 inches in height or taller.
AquaDunk on the Disney Magic: Designed for adults and children who are at least 48 inches in height or taller
Aqualab on the Disney Magic: Designed for children ages 3 years of age and older (swim diapers are prohibited)
Nephews' Splash Zone on the Disney Magic: Designed for children ages 3 years of age and younger (swim diapers are required)
Mickey's Splash Zone and Nemo's Reef: Designed for children ages 3 years of age and younger (swim diapers are required)
Twist 'n' Spout Slide on the Disney Magic: Riders must be between 38 inches and 64 inches in height
Pools on Disney Cruise Line ships.
Important Information About Swim Diapers
The United States Public Health Service requires that only children who are toilet trained are permitted to enter swimming pools and spas aboard cruise ships.
Children who are not toilet trained are welcome to enjoy Nephews' Splash Zone on the Disney Magic, Nemo's Reef on the Disney Dream and the Disney Fantasy, or Mickey's Splash Zone on the Disney Wonder where they must wear swim diapers. Swim diapers are not permitted in Goofy's Pool, Donald's Pool, Quiet Cove Pool, AquaDuck, AquaDunk, AquaLab, hot tubs or spas.
Please note that snorkel equipment and swim goggles are not permitted in pools, hot tubs or water-play areas.